util: Add API to query call icon names

This can later be used in the call history or in the call details
instead of always constructing the name repeatedly in private functions.

src/util.h

@@ -142,6 +142,8 @@ const char* get_protocol_from_address (const char *target);
 const char* get_protocol_from_address_with_fallback (const char *target);
 
 gboolean dtmf_tone_key_is_valid (char key);
+const char *get_call_icon_symbolic_name (gboolean inbound,
+                                         gboolean missed);
 
 G_END_DECLS